Local and Global Variables in C++

In C++, the scope defines the region within which a variable can be accessed. So, if broadly speaking, there are three main places where variables can be declared and accessed:

• Inside a function or a block, which are called local variables.
• In the definition of function parameters, which are called formal parameters.
• Outside of all functions, which are called global variables.
  

Local Variables

Local variables are the variables that are defined inside a function, method, or block of code within curly braces {}. These variables cannot be accessed or used outside of that function or block.

Example

Here is the following example code, showcasing the local variables inside a function. where these variables will be accessible in their function or scope only and can't be accessed out of its scope:

#include <iostream>
using namespace std;

void calculateSum() {
    int x = 15;    // local variable declaration
    int y = 25;    // local variable declaration
    int sum = x + y;
    cout << "Sum is: " << sum << endl;
}

int main() {
    calculateSum();  // calling a function
    // cout << x; this will be wrong because the x variable is not directly accessible here out of its local function scope
    return 0;
}

Output

Sum is: 40

Example

Here is the following example code, showcasing the Local variables inside a block. where these variables will be accessible till their block or scope only:

#include <iostream>
using namespace std;

int main() {
    int a = 5;  // this is local variable to main() function
    
    if (a > 0) {
        int b = 10;  // local variable to this if-block
        cout << "Inside block: a = " << a << ", b = " << b << endl;
    }
    
    cout << a <<endl;
    // cout << b; but b is not accessible outside the if-block
    return 0;
}

Output

Inside block: a = 5, b = 10
5

Global Variables

These are the variables, which are defined outside of all the functions, usually at the top of the program. These global variables hold their values throughout the lifetime of your program and can be accessed by any function or block.

Example

Here is the following example showcasing how the global variable is accessed and used in the main() function:

#include <iostream>
using namespace std;

// Global variable declaration:
int g;

int main () {
   // Local variable declaration:
   int a, b;

   // initialization of local variables
   a = 10;
   b = 20;
   g = a + b;  // using local variables to update global variable

   cout << g;
   return 0;
}

Output

30

Variable Shadowing

In C++, when both global and local variables have the same name, then the local variable takes precedence within its scope. This means the value of the local variable will be used within that scope. In case if you want to access a global variable, you need to use the scope resolution operator (::).

Example

Here is the following example showcasing how, by default, a local variable is used when both local and global variables have the same name. So, to access the global variable, we used the scope resolution operator.

#include <iostream>
using namespace std;
// Global variable declaration:
int g = 20;
int main () {
   // Local variable declaration:
   int g = 10;

   cout << g << endl;   // it will call local variable
   cout << ::g << endl; // using the scope resolution operator will give you access to call the value of the global variable
   return 0;
}

Output

10
20